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Sudoku

Sudoku is a pure logic puzzle that uses the numerals 1,...,9 as markers, but involves no math or arithmetic.
It was invented in the 1970's in Michigan, but took off first in Japan, later returning to the US and the UK.
The Sudoku grid is 9x9, and a given puzzle has some of the squares already filled in. The goal is to fill
in the blank squares in such a way that each numeral occurs just once in every row and column, and
in every subsidiary 3x3 grid marked with heavy lines. As in all the other Strong Brain tasks, pushing
yourself for speed takes increased concentration which builds mental muscle.